1. Field of the Invention
The present invention relates to a master terminal system for a star network wherein a master station transmits a continuous signal carrying identical data to a plurality of terminal stations and the terminal stations transmit respective signals to the master station according to a time-division multiple access procedure, and more particularly, to a master terminal system for a star network in which the master station detects a deviation of multiplexing position of the signals transmitted from the terminal stations by using an inexpensive device, without requiring special firmware.
2. Description of the Related Art
Recently, among other satellite communications, star network for satellite communications has attracted attention wherein a signal from a single master station is received equally by a plurality of terminal stations in the network, and construction of practical systems is being attempted. This is because in satellite communications, divergent geographical conditions of terminal stations need not be taken into account on the part of the master station, facilitating construction of extensive broadcast communication systems.
In many one-to-N star network systems for satellite communications composed of one master station and N terminal stations, the master station transmits a signal in the form of a continuous wave (single carrier wave) to the terminal stations in broadcast mode, whereas each terminal station transmits a signal to the master station in certain multiplex mode. Particularly, in cases where the number of terminal stations is large (e.g., N.gtoreq.100), a time-division multiple access (TDMA) procedure wherein time-division multiplexing is effected using an identical carrier frequency for intermittent transmission is more suitable for the transmission of signals from the terminal stations to the master station and is easier to introduce than other procedures such as frequency-division multiplexing and code multiplexing.
Generally, in satellite communications, the transmission distance between the satellite and an earth station varies due to attitude control operations of the satellite in an orbit, causing variation of received clock frequency as a result of Doppler shift. Thus, data cannot always be received at stable timing. Taking the circumstances into account, satellite communications utilize a synchronizing word (hereinafter referred to as "unique word") which is inserted in a predetermined position of transmitted data, so that the receiving side can detect the unique word and use the detected position as a reference when detecting the data. Unique word detection methods include an open type wherein received signals are monitored at all times to detect a unique word, and an aperture type wherein a time zone during which a unique word may probably be received is predicted by suitable means, a monitoring time frame called aperture window is set corresponding to the predicted time zone, and the received signal is monitored only within the time frame to detect input of a unique word.
In the time-division multiple access (TDMA) procedure, each terminal station sends its own transmit signal, is allotted a time slot position for multiplexing, and transmits a unique word and data within a time slot period assigned thereto, according to the precision prescribed by the network. It is, therefore, easy for the master station to predict the unique word position in each time slot, and thus the master station uses the aperture method for the detection of unique words.
The width (time width) of the aperture window varies depending on the communication method, clock frequency, frequency stability of transmitting stations (particularly, that of the master station in a network), control period, quantization error associated with digital processing, and other factors, and is usually determined in consideration of these factors.
Meanwhile, in cases where the line communication speed is high, the line information efficiency (the ratio of true information to the total information transmitted through the line) lowers if the width of the aperture window is increased. Accordingly, the width of the aperture window is reduced, and a dynamic control is carried out on a real-time basis. In cases where the line communication speed is low, on the other hand, the width of the aperture window can be increased to a considerable extent, whereby proper operation is ensured even if the position of a unique word in a transmitted signal is slightly deviated. Namely, the position of the unique word in a signal transmitted from each of the terminal stations is slightly deviated from its proper position due to geographical conditions of the terminal stations. Thus, a unique word detection method using a wide aperture window is effective for a transmission system (i.e., a fixed transmission control system) wherein measurement of the aforesaid deviation and adjustment of the transmission timing are carried out at the time of installation of terminal stations, and signals are transmitted thereafter ignoring the deviation of the unique word position from the proper position.
In one-to-N star networks for satellite communications, the aforementioned fixed transmission control technique is employed because the information transmitted from a terminal station to the master station includes only control information and monitor information and the information efficiency is not an important factor in determining the transmission method to the used, and also because terminal stations should preferably be inexpensive.
In conventional one-to-N star network for satellite communications using the fixed transmission control technique, the master station is, if it detects a deviation of the unique word position originated by a terminal station from the proper position during operation, unable to cope with the situation on a real-time basis, and thus has neither detecting means nor notifying means. Further, it is not practical in view of cost to provide the master station with firmware for the purpose of only temporarily detecting the deviation at the time of installation of terminal stations. Under the circumstances, a determination as to whether a signal transmitted from a terminal station at the time of installation is properly multiplexed at a proper position is conventionally made at the master station by using a measuring instrument such as a synchroscope.
This confirmation work using a measuring instrument such as a synchroscope, however, can be performed only by an expert who is accustomed to the measuring instrument, has knowledge of monitor points in the master station, and is skilled in the technical evaluation of measurement results, etc. Moreover, each time a terminal station is installed, such a skilled person must carry the measuring instrument to the master station.
Further, in cases where the period (control frame length) at which each terminal station transmits a local signal is long, e.g., several tens of seconds, such a long-period signal may not be displayed at the synchroscope, making the confirmation work difficult.
Furthermore, where a determination as to whether the position of a unique word transmitted from a particular terminal station is deviated from the proper position is made at the master station during operation, and not at the time of installation of the terminal station, a measuring instrument such as a synchroscope must inevitably be connected to the master station, lowering the maintainability of networks under operation.